MESOSCALE DISCUSSION 0684
   NWS STORM PREDICTION CENTER NORMAN OK
   0654 AM CDT SUN MAY 03 2009
   
   AREAS AFFECTED...PORTIONS E TX...LA...SWRN MS.
   
   CONCERNING...SEVERE THUNDERSTORM WATCH 231...
   
   VALID 031154Z - 031400Z
   
   THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 231
   CONTINUES.
   
   LEADING EDGE OF FORCED ASCENT ACCOMPANYING BAND OF TSTMS THAT HAD
   BEEN BEHIND COLD FRONT OVER CENTRAL TX IS CROSSING INTO HIGH-THETAE
   LOW-LEVEL WARM SECTOR ATTM.  RADAR BASE DATA AND IR IMAGERY
   ACCORDINGLY INDICATE THAT MCS IS ENLARGING BY BACKBUILDING SWD...AND
   ALSO BECOMING BETTER ORGANIZED.  EXPECT INCREASINGLY INTENSE AND
   WIDESPREAD DAMAGING WIND POTENTIAL -- PERHAPS A DERECHO -- AS
   COMPLEX MOVES EWD APPROXIMATELY 50 KT INTO MORE UNSTABLE WARM
   SECTOR.
   
   AT 1130Z...PREFRONTAL OUTFLOW BOUNDARY WAS ANALYZED FROM CENTRAL MS
   NEAR JAN...WSWWD ACROSS LA SALLE PARISH LA...THEN WSWWD ACROSS NRN
   PORTION TOLEDO BEND LAKE AND INTERSECTING NEWER MCS OVER RUSK COUNTY
   TX.  AHEAD OF FRONT AND S OF OUTFLOW BOUNDARY...SHALLOW LAYER OF
   DIABATICALLY COOLED/NEAR-SFC AIR MAY DAMPEN DOWNDRAFT STRENGTH
   SOMEWHAT COMPARED TO MAX POTENTIAL...AT LEAST INITIALLY. 
   NONETHELESS...A FEW REPORTS OF DOWNED TREES HAVE BEEN RECEIVED SINCE
   1054Z FROM BRAZOS/HOUSTON COUNTIES.  SEVERAL FACTORS INDICATE
   WELL-ORGANIZED DAMAGING WIND EVENT IS POSSIBLE ACROSS SWATH OF E TX
   AND LA...AND PERHAPS FARTHER E...
   
   1. TRANSLATIONAL MOTION OF COLD POOL THAT HAS BEEN AUGMENTED BY
   POSTFRONTAL AIR ALREADY REACHES SVR CRITERIA...AND IS ENCOUNTERING
   2. INCREASINGLY SFC-BASED EFFECTIVE LIFTED PARCELS ALONG AND S OF
   OUTFLOW BOUNDARY...AND IN A FEW MORE HOURS...
   3. DIURNAL HEATING/DESTABILIZATION OF FOREGOING AIR MASS THAT WILL
   BOOST BOTH CAPE AND DCAPE...AMIDST
   4. FAVORABLE EFFECTIVE SHEAR VALUES OF 45-55 KT...WITH MEAN WIND AND
   DEEP-LAYER SHEAR VECTORS ALIGNED NEARLY PERPENDICULAR TO MCS.
   
   MAIN UNCERTAINTY IS POSSIBILITY THAT...BECAUSE OF FAST MCS
   MOTION...TSTMS MAY MOVE INTO SUBSTANTIALLY STRONGER MLCINH OVER
   S-CENTRAL/SERN LA BEFORE MUCH DIABATIC DESTABILIZATION CAN OCCUR. 
   OVER ANOTHER COUPLE OF HOURS...THESE TRENDS...AS WELL AS MCS
   GROWTH/MOVEMENT...WILL BE MONITORED FOR POSSIBLE WW ISSUANCE E OF WW
   231.
   
   ..EDWARDS.. 05/03/2009
   
   
   ATTN...WFO...JAN...LIX...LCH...SHV...HGX...
   
   LAT...LON   30389623 30929561 31869497 32759464 32719385 32409059
               31019047 30409142 29919467 30389623